Iceburg lettuce
Celery
They provide very little useful nutrition or fiber. They are fillers, a way to add variety but not nutrition.

Spinach
Chard
Beets
Rhubarb
Oxalic acid blocks the absorption of calcium. These foods cause the turtle to become calcium deficient even if a good source of calcium is provided which can result in soft shell or MBD (metabolic bone disease) which is fatal.